A turbomachine having a rotating shaft must provide some means for supporting and positioning the rotatable shaft while the shaft is rotating about its longitudinal axis. In general, it is desirable for a turbomachine to also provide some means for damping radial movement of the shaft. A general practice is to provide a bearing to support and position the rotating shaft and to permit free rotation of the shaft with minimum friction. One type of bearing simply provides a low friction surface adjacent the shaft in order to support the shaft. The most common low friction surface is a lubricant film, such as an oil film. The method of supplying the lubricant and the quantity of lubricant which is fed to the bearing will often be the greatest factors in establishing the performance characteristics of the bearing. For example, if no lubricant is present, the shaft and bearing will rub against each other in the dry state. However, where lubricant is present between the surfaces in even small quantities, the coefficient of friction between the surfaces will be reduced. If an abundance of lubricant is fed to the bearing so that there is an excess flowing out of the bearing, it is possible to provide a self-generating pressure film in the clearance space between the shaft and bearing. Such pressure film can be sufficient to dampen radial movement of the shaft and to keep the surfaces of the bearing from rubbing against each other. Failure to adequately dampen the radial movement of the shaft can result in a wide variety of rotor vibration problems.
In an attempt to increase the level of damping in rotor systems, squeeze film damper bearings have been developed. A traditional practice is to provide seals that maintain a pressurized lubricant film, or squeeze film, in the clearance space between the bearing cage and housing to dampen radial movement of the shaft. The initial designs of squeeze film damper bearings provided a cylindrical design that provided a squeeze film radially between a cylindrical radially outer surface of a cage and a cylindrical radially inner surface of a housing that afforded the dampening effect. However, there was some difficulty in adjusting the amount of dampening provided by these initial designs as a result of their cylindrical design. Thus, bearings have been designed to provide a squeeze film damper that is adjustable to afford the desired degree of dampening.
U.S. Pat. No. 4,392,751 to Ida et al. is an example of a bearing designed to provide an adjustable squeeze film damper. The damped bearing device of U.S. Pat. No. 4,392,751, which utilizes a fluid film between inclined surfaces, has a mating taper between a bearing and a bearing back-up member that provides an adjustable squeeze film damper. The bearing is connected to a housing by a resilient bearing support. The bearing back-up member is disposed between the housing and the bearing. The clearance between the bearing and the bearing back-up member is altered by adjusting the axial position of the bearing back-up member with respect to the bearing, the resilient bearing support, and the stationary housing. In order for the bearing back-up member to be adjustable, a radial clearance between the bearing back-up member and the housing is required. This clearance can lead to vibrational problems if the retaining bolts fail to constrain the bearing laterally. Additionally, the bolts are subjected to cyclic loading which can cause them to fail in fatigue. In addition, the presence of a bearing back-up member results in a complex design which is subject to dimensional tolerancing problems. Thus, there is a need in the art for a bearing that provides a squeeze film damper for supporting the shaft and for dampening vibration of the shaft during shaft rotation while at the same time not comprising reliability.
There is also a need in the art for a bearing that optionally can function as a traditional bearing and that optionally can provide a squeeze film damper for supporting the shaft and for dampening vibration of the shaft during shaft rotation. There is also a need in the art for such a bearing wherein the squeeze film damper is adjustable.